Conventionally, as a linear guide unit of the above type, there is known the one that has a structure comprising a orbital rail arranged on a stationary portion such as a bed and having ball rolling contact surfaces and a slider fastened to the orbital rail through a number of balls and movable along the rail which supports a movable body such as a table.
Further, the slider comprises a mobile block having ball load rolling contact surfaces opposing to the ball rolling contact surface of the orbital rail through the balls and ball return holes parallel to the ball load rolling contact surfaces and capable of moving along the orbital rail following the rolling of the balls, a pair of covers having change-direction paths for establishing communication between the ball load rolling contact surfaces and the ball return holes and ball retainers attached to the mobile block so as to prevent the falling of the balls from the ball load rolling contact surfaces and the end portions of the ball return holes are connected by the change-direction path when the covers are fixed to the front and rear end surfaces of the mobile block, respectively, thereby completing an endless circulation path for the balls in the slider.
In the case of the slider of the conventional rectilinear guide unit of the above-described structure, the process of drilling the ball return holes and the fixing of the ball retainers to the mobile block becomes necessary resulting in taking much time and labor for its manufacture so that the present applicant proposed a method of manufacture of the slider by making use of extrusion molding of a synthetic resin (refer to Japanese Unexamined Patent Publication No. 317762/1995).
More concretely, the above-proposed method is such that a synthetic resin is padded onto the metallic block main body having the above-mentioned load rolling contact surfaces subjected to cutting so that the ball return holes and the ball retainers are molded integral with the block main body and the ball guide sections on the side of the inner peripheral surface of each of the change-direction paths which have hitherto been formed in the covers are also molded on the end surfaces of the block main body to thereby manufacture the movable block. Besides the advantage of being able to manufacture a mobile block of a complicated shape in a simple manner, this proposed method has also such advantage that an endless ball circulation path provided with ball return holes, inner peripheral side ball guide sections and ball retainers formed continuous with one another can be formed with a synthetic resin thereby smoothing the circulation of the balls.
Now, where such endless ball circulation path is continuously formed with the synthetic resin, it is usual that the molten resin immediately after injection-molding solidifies as it contracts toward a direction parallel to the ball return holes but since such endless circulation path itself is so formed as to surround the block main body, in the case of the mobile block which has been subjected to molding, the synthetic resin forming the endless ball circulation path binds the block main body tightly and a tensile stress remains in the molded synthetic resin. Consequently, there has arisen the problem that when the block main body has front and rear corners near the boundary of the ball return holes and the inner peripheral side ball guide sections formed of the synthetic resin, the tensile stress concentrates on the corners causing the synthetic resin to crack thereby hindering the smooth rolling of the balls in the endless ball circulation path.
Further, there has also been the problem that when the block main body has corners formed of synthetic resin, the flow of the molten resin is hindered at the time of injection-molding and the fluidity of such molten synthetic resin becomes insufficient so that a weld line is generated in the synthetic resin forming the endless ball circulation path and the smooth rolling of the balls is hindered because of the presence of such weld line.
In addition, due to the fact that when the block main body has corners formed of synthetic resin, the flow of the molten resin is hindered due to the presence of these corners, the thickness of the synthetic resin at the ball return holes and the ball retaining sections tends to become non-uniform and it has not been able to avoid the generation of deformation of the ball return holes and the ball retaining sections in the cooling process after molding. Accordingly, there has been the problem that the smooth rolling of the balls is hindered because of this point, too.
Now, in order to allow the balls rolling in the endless ball circulation path to circulate smoothly along a predetermined track, it is necessary to prevent each of the balls rolling within the endless ball circulation path from unsteadily moving right and left by reducing the gap between the ball and the inner wall of the endless circulation path to a minimum. Accordingly, in order to secure the smooth movement of the slider with respect to the orbital rail, it is necessary to mold the endless ball circulation path to a predetermined dimensional accuracy without fail so that a special consideration has been required for padding a synthetic resin to the metallic block main body by injection-molding.